# C

> Write safe C code by avoiding memory corruption, buffer overflows, and undefined behavior traps.

- Skill: `luokai0/c` (Agent Skill, multi-file: 15 files)
- Install (CLI): `npx skillmds@latest add luokai0/c`
- Raw SKILL.md: https://api.skillmd.com/api/skills/luokai0/c/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools, AI & ML
- Tags: Buffer Overflow, Malloc, Memory Safety, Pointers, Strings, Undefined Behavior
- Author: luokai0 (https://skillmd.com/u/luokai0)
- Updated: 2026-08-22
- Page: https://skillmd.com/skills/luokai0/c

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## Quick Reference

| Topic | File |
|-------|------|
| malloc/free, leaks, double free | `memory.md` |
| Null, dangling, pointer arithmetic | `pointers.md` |
| Null terminator, buffer overflow | `strings.md` |
| Integer overflow, signed/unsigned | `types.md` |
| Macro traps, include guards | `preprocessor.md` |
| Common undefined behavior | `undefined.md` |

## Critical Rules

- `malloc` returns `void*` — cast required in C++, optional in C but check for NULL
- `free(ptr); ptr = NULL;` — always null after free to prevent double-free
- `sizeof(array)` in function gives pointer size, not array size — pass length separately
- `char str[5] = "hello";` — no room for null terminator, UB when used as string
- `strcpy` doesn't check bounds — use `strncpy` and manually null-terminate
- Signed overflow is UB — compiler can optimize assuming it never happens
- `i++ + i++` is UB — no sequence point between modifications
- Returning pointer to local variable — dangling pointer, UB on use
- `#define SQUARE(x) x*x` — `SQUARE(1+2)` = `1+2*1+2` = 5, not 9
- `memcpy` with overlapping regions — use `memmove` instead
- Uninitialized variables — contain garbage, UB if used
- Array out of bounds — no runtime check, silent corruption or crash